Conventionally, a general purpose semiconductor laser used for recording in an image forming apparatus has the so-called APC (Automatic Power Control) function to control an emitted light amount of the semiconductor laser. When the APC function is performed, the emitted light amount is periodically detected by a photo sensor disposed in the vicinity of the semiconductor laser, and a detection signal in accordance with the detected light amount is output from photo sensor to feed back to the driving circuit which drives the semiconductor laser. By having this function, the emitted light amount is controlled to produce a desired amount of laser light.
FIG. 15 is a block diagram showing a configuration of a conventional semiconductor laser driving device.
As shown in FIG. 15, when a photo diode PD receives an emitted laser light from a semiconductor laser such as a laser diode LD, the photo diode PD outputs a monitor current Im in accordance with the received light amount of the laser light.
Then, the monitor current Im is converted into a monitoring voltage Vm by an I/V converting circuit 101. The monitoring voltage Vm is compared with a predetermined reference voltage by a comparison circuit 102. Then, the comparison circuit 102 outputs a signal indicating the voltage difference. A sample hold circuit 103 samples and holds the voltage of the signal transmitted from the comparison circuit 102 in a non-imaging region at the timing of an external APC signal. Then, the sample hold circuit 103 transmits the held voltage to a driving current setting circuit 104 as the driving voltage in an imaging region. The driving current setting circuit 104 generates a driving current in accordance with the received driving voltage and supplies the generated driving current to the laser diode LD. In this configuration, the comparison circuit 102 and the sample hold circuit 103 constitute an APC circuit.
However, it is known that the service lifetime of the semiconductor laser LD is generally shorter than that of the other parts. Therefore, the semiconductor laser LD may be exchanged before becoming out of order by detecting a time-dependent deterioration of the semiconductor laser LD and issuing an alarm indicating that the deterioration is detected.
FIGS. 16 and 17 show typical cases where the characteristics of a semiconductor laser LD are deteriorated.
As shown in FIGS. 16 and 17, when the semiconductor laser LD deteriorates over time or for another reason, the threshold current Ith of the semiconductor laser LD may be increased, and as a result, the slope of the emitted light amount with respect to the driving current with the semiconductor laser LD being deteriorated is obviously more inclined than that before its being deteriorated. Because of this feature, in order to obtain the same emitted light amount from the semiconductor laser LD after the semiconductor laser LD has become deteriorated, a larger driving current Iop is required to be supplied to the semiconductor laser LD than before the semiconductor laser LD is deteriorated. When the semiconductor laser LD is further deteriorated, the driving current required to be supplied to the semiconductor laser LD may be greater than an absolute maximum rated value Imax, and then the semiconductor laser LD may malfunction.
In a case where the deterioration of the semiconductor laser LD is not detected until the semiconductor laser LD becomes unable to emit light, there may be no preparation to replace the semiconductor laser LD until the semiconductor laser LD becomes unable to emit light, either. As a result, the deteriorated semiconductor laser LD may not be replaced almost immediately after the semiconductor laser LD becomes unable to emit light. Further, there may be another case where the semiconductor laser LD becomes unable to emit light during its manufacturing process. In any case, when the deterioration of the semiconductor laser LD can be detected at any early stage, it may become possible to replace the semiconductor laser LD more easily with lower cost.
To that end, as shown in FIG. 18, a method is disclosed in which a current flowing through the semiconductor laser LD is detected using a resistor 111 and an amplifying circuit 112, the resistor 111 being connected in series with the semiconductor laser LD. The detected current value is compared with a predetermined reference current value in a comparison circuit 113. Then, when the detected current value as the comparison result is greater than the predetermined reference current value, the comparison circuit 113 outputs a signal indicating that the semiconductor laser LD is deteriorated. (see, for example, Patent Document 1)
[Patent Document 1] Japanese Laid-Open Patent Application No. 2000-280522